Fast frequency calculator

1. An apparatus, comprising: a capacitor; a reference voltage selectively communicatively coupled to the capacitor by a first switch; an input signal; and a frequency calculation circuit communicatively coupled to the capacitor, the first switch, and the input signal, and configured to: select a capacitance value for the capacitor; charge the capacitor with the reference voltage by at least operating the first switch; discharge the capacitor to a threshold voltage by at least further operating the first switch; and based on a comparison of time to discharge the capacitor to the threshold voltage with a clock cycle of the input signal, determine a frequency of the input signal.

2. The apparatus of claim 1 , wherein the frequency calculation circuit is further configured to select a subsequent capacitance value for the capacitor based upon the comparison of time to discharge the capacitor with the clock cycle of the input signal.

3. The apparatus of claim 2 , wherein the frequency calculation circuit is further configured to: charge the capacitor with the reference voltage after setting the capacitor with the subsequent capacitance value; discharge the capacitor to the threshold voltage after setting the capacitor with the subsequent capacitance value; and determine the frequency of the input signal further based on a comparison of the clock cycle of the input signal with time to discharge the capacitor to the threshold voltage after setting the capacitor with the subsequent capacitance value.

4. The apparatus of claim 1 , wherein the frequency calculation circuit is further configured to, until a search threshold is reached: iteratively select and set subsequent capacitance values for the capacitor based upon a previous comparison of time to discharge the capacitor with the clock cycle of the input signal; charge and discharge the capacitor after each capacitance value is iteratively selected and set for the capacitor; and compare the clock cycle of the input signal with a given time to discharge the capacitor to the threshold voltage given a subsequent capacitance value.

5. The apparatus of claim 4 , wherein the frequency calculation circuit is configured to perform a binary search to iteratively select and set subsequent capacitance values.

6. The apparatus of claim 4 , wherein the frequency calculation circuit is further configured to determine the frequency of the input signal further based on the comparisons of the clock cycle of the input signal with the given times to discharge the capacitor to the threshold voltage given the subsequent capacitance values.

7. The apparatus of claim 1 , further comprising a comparison circuit communicatively coupled to the frequency calculation circuit, the voltage source, and the capacitor, and configured to: determine discharge of the capacitor to the threshold voltage by a comparison of an instant voltage of the capacitor while discharging with a divided voltage of the reference voltage; and provide the discharge of the capacitor to the frequency calculation circuit.

8. The apparatus of claim 1 , wherein the frequency calculation circuit is implemented as a successive approximation register.

9. The apparatus of claim 1 , wherein the capacitor is a variable capacitor and the frequency calculation circuit is configured to set the capacitance of the variable capacitor.

10. The apparatus of claim 1 , wherein the determination of the frequency of the input signal is based upon an association of the time to discharge the capacitor to the threshold voltage with the capacitance value for the capacitor.

11. The apparatus of claim 1 , wherein the frequency calculation circuit is further configured to determine the frequency of the input signal based on an inverse of time to discharge the capacitor to the threshold voltage.

12. A method, comprising: selecting a capacitance value for a capacitor; charging the capacitor with a reference voltage; discharging the capacitor to a threshold voltage; and based on a comparison of time to discharge the capacitor to the threshold voltage with a clock cycle of an input signal, determine a frequency of the input signal.

13. The method of claim 12 , further comprising selecting a subsequent capacitance value for the capacitor based upon the comparison of time to discharge the capacitor with the clock cycle of the input signal.

14. The method of claim 13 , further comprising charging the capacitor with the reference voltage after setting the capacitor with the subsequent capacitance value; discharging the capacitor to the threshold voltage after setting the capacitor with the subsequent capacitance value; and determining the frequency of the input signal further based on a comparison of the clock cycle of the input signal with time to discharge the capacitor to the threshold voltage after setting the capacitor with the subsequent capacitance value.

15. The method of claim 12 , further comprising, until a search threshold is reached: iteratively selecting and setting subsequent capacitance values for the capacitor based upon a previous comparison of time to discharge the capacitor with the clock cycle of the input signal; charging and discharging the capacitor after each capacitance value is iteratively selected and set for the capacitor; and comparing the clock cycle of the input signal with a given time to discharge the capacitor to the threshold voltage given a subsequent capacitance value.

16. The method of claim 15 , further comprising performing a binary search to iteratively select and set subsequent capacitance values.

17. The method of claim 16 , further comprising determining the frequency of the input signal further based on the comparisons of the clock cycle of the input signal with the given times to discharge the capacitor to the threshold voltage given the subsequent capacitance values.

18. The method of claim 12 , further comprising determining discharge of the capacitor to the threshold voltage by a comparison of an instant voltage of the capacitor while discharging with a divided voltage of the reference voltage.

19. The method of claim 12 , wherein the determination of the frequency of the input signal is based upon an association of the time to discharge the capacitor to the threshold voltage with the capacitance value for the capacitor.

20. An integrated circuit device, comprising: a variable capacitor; a reference voltage selectively communicatively coupled to the capacitor by a first switch; an input signal; and a frequency calculation circuit communicatively coupled to the capacitor, the first switch, and the input signal, and configured to: select a first capacitance value for the variable capacitor; charge the variable capacitor with the reference voltage by at least operating the first switch; discharge the variable capacitor to a threshold voltage by at least further operating the first switch; compare completion of discharging the variable capacitor to the threshold voltage with completion of a clock cycle of the input signal; based on a determination that the variable capacitor completed discharging to the threshold voltage finished before completion of the clock cycle of the input signal, selecting a second capacitance value of the variable capacitor, wherein the second capacitance value of the variable capacitor is higher than the first capacitance value; based on a determination that the variable capacitor completed discharging to the threshold voltage finished after completion of the clock cycle of the input signal, selecting a second capacitance value of the variable capacitor, wherein the second capacitance value of the variable capacitor is lower than the first capacitance value; and determining a frequency of the input signal based upon the second capacitance value.